K rather than Kc

$\ln K = -\frac{\Delta H^{\circ}}{RT} + \frac{\Delta S^{\circ}}{R}$

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Jessa Maheras 4F
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Joined: Fri Aug 02, 2019 12:16 am

K rather than Kc

In the book, it says that in order to use the Van't Hoff Eq for runs involving gases, you have to convert from K too Kc. What is the difference between the two terms?

Kishan Shah 2G
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Re: K rather than Kc

This is a good question. When I was doing my calculations I used the same value for both but my calculations could be wrong. Does anyone know?

alicechien_4F
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Re: K rather than Kc

You have to convert from Kp to Kc because the values are different. I believe Kp and Kc are only the same when the number of moles of gas on the reactant and products sides are equal (correct me if I'm wrong!). Van't Hoff's equation relates Kc to change in temperature.

Julie_Reyes1B
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Re: K rather than Kc

In the book, K without a subscript refers to Kp, whereas in Dr. Lavelle's class, both terms will be identified using the subscripts.

Marni Kahn 1A
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Re: K rather than Kc

K is in terms of pressure, whereas Kc is in terms of concentration/molarity.

205405339
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Re: K rather than Kc

K=(RT)^(change in n) x Kc

Robin Cadd 1D
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Re: K rather than Kc

Jessa Maheras 4F wrote:In the book, it says that in order to use the Van't Hoff Eq for runs involving gases, you have to convert from K too Kc. What is the difference between the two terms?

For gases, you automatically assume that K is Kp. Converting K to Kc is just converting K to Kp

Jessa Maheras 4F
Posts: 121
Joined: Fri Aug 02, 2019 12:16 am

Re: K rather than Kc

Julie_Reyes1B wrote:In the book, K without a subscript refers to Kp, whereas in Dr. Lavelle's class, both terms will be identified using the subscripts.

Thank you Julie that makes sense!!

Jessa Maheras 4F
Posts: 121
Joined: Fri Aug 02, 2019 12:16 am

Re: K rather than Kc

Robin Cadd 1D wrote:
Jessa Maheras 4F wrote:In the book, it says that in order to use the Van't Hoff Eq for runs involving gases, you have to convert from K too Kc. What is the difference between the two terms?

For gases, you automatically assume that K is Kp. Converting K to Kc is just converting K to Kp
Oh alright, thanks!

KHowe_1D
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Re: K rather than Kc

Kp is in terms of pressure while Kc is in terms of molarity (concentration). I would say just during calculations to specify with the subscript.

Nawal Dandachi 1G
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Joined: Sat Sep 28, 2019 12:16 am

Re: K rather than Kc

I always assume they are the same, but I guess when you are referring to gases, K is Kp, and Kp is not equal to Kc.

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